1. Field of the Invention
This invention relates to the production, purification and polymerization of aromatic dicarboxylic acids for use in the preparation of polyesters. In particular, the invention relates to processes for producing and purifying crude terephthalic acid and 2,6-naphthalenedicarboxylic acid and for using the purified acids in the production of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).
2. Description of the Prior Art
Polyesters are polymers typically prepared by polycondensation reactions starting from polycarboxylic acids and polyols. The polyesters of greatest commercial interest are those based on the reaction products of terephthalic acid and 2,6-naphthalenedicarboxylic acid with aliphatic diols, with the preferred diol being ethylene glycol. The first commercial polyester was polyethylene terephthalate (PET). However, more recently significant attention has been focused towards polyethylene naphthalate (PEN), because fibers and films made from PEN have improved strength and thermal properties relative to, for example, fibers and films made from PET. High strength fibers made from PEN can be used to make tire cord, and films made from PEN are advantageously used to manufacture magnetic recording tape and electronic components. Also, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, particularly so-called xe2x80x9chot fillxe2x80x9d type food containers. Polyesters made from mixtures of terephthalic acid and 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate also have been found to have unique and desirable properties such as resistance to gas diffusion, making them suitable for manufacturing, for example, beverage containers or other containers for food products, including containers for beer.
Polyester resin is most often presently prepared by forming a slurry of the purified aromatic dicarboxylic acid, or the dimethylester of the aromatic dicarboxylic acid, and ethylene glycol, in the presence of an esterification catalyst such antimony, and subjecting the mixture to successively higher temperature and lower pressures to drive out the condensation products; and then, in the presence of a polyesterification catalyst the excess ethylene glycol is removed under reduced pressure to bring the molecular weight to the desired range. The current polyester production process involves at least three steps. In the first step, esterification of the acid with excess glycol (or transesterification if the methyl esters are used), the bulk of the water or methanol is removed. The diglycol ester then passes to the second, prepolymerization step to form intermediate molecular weight oligomers before passing to the third, melt polyesterification step operated at low pressure and high temperature. For some applications requiring higher melt viscosity a further solid-state polymerization is practiced.
As will be discussed in more detail below, current processes for the preparation of terephthalic acid and 2,6-naphthalenedicarboxylic acid involve catalytic oxidation of p-xylene or 2,6-dimethylnaphthalene and provide a crude oxidation product which contains, as major impurities, mono-carboxylic acids, tricarboxylic acids, such as trimellitic acid, and aldehydes produced as oxidation by-products, together with residues, such as cobalt, manganese and bromine, derived from the oxidation catalyst. However, it is well known that, when used as starting materials for the manufacture of polyester fibers and films, dicarboxylic acids must achieve a high degree of purity, since the presence of contaminants, even in minute amounts, can have deleterious effects upon the quality of the resulting resin. For instance, in the case of terephthalic acid, monocarboxylic acid oxidation intermediates, such as p-toluic acid and/or 4-carboxybenzaldehyde, may react with ethylene glycol when present in the polycondensation reaction mixture and therefore act as chain stoppers, with the consequence that the melting point and strength of the resulting polyester may be substantially and undesirably lowered. Moreover, the impurities present in the crude acid can result in discoloration of the PET or PEN resin, as well as mold staining during the molding process, thereby decreasing the transparency of the molded products and hence lowering the product quality.
Thus, in order to obtain high-quality, high molecular weight PET and PEN, the crude dicarboxylic acid needs to be purified before it is used as a starting material for preparing polyesters. Several processes have been proposed for the purification of crude terephthalic acid and naphthalene dicarboxylic acid and are described below.
For example, U.S. Pat. No. 4,317,924 discloses a process for purifying crude terephthalic acid by treating a suspension of the crude acid in an aqueous solution of a water-soluble heavy metal salt with nitrogen and/or hydrogen in the presence of a supported noble metal catalyst under conditions sufficient to reduce the 4-carboxybenzaldehyde impurity without significant reduction of the terephthalic acid. The treated solution is then separated from the catalyst and the purified crystalline terephthalic acid is recovered by crystallization.
U.S. Pat. Nos. 6,100,374 and 5,872,284 describe a process of purifying crude naphthalene dicarboxylic acid comprising the steps of mixing crude naphthalene dicarboxylic acid and an ethylene glycol aqueous solution, heating the resulting mixture to esterify part of the naphthalene dicarboxylic acid and thereby give a naphthalene dicarboxylic acid ester and dissolving the naphthalene dicarboxylic acid ester in the ethylene glycol aqueous solution; then contacting impurities, which are contained in the crude naphthalene dicarboxylic acid and capable of being hydrogenated, with hydrogen in the presence of a hydrogenation catalyst to hydrogenate the impurities and dissolving the hydrogenated impurities in the ethylene glycol aqueous solution; and subsequently crystallizing the naphthalene dicarboxylic acid ester and separating the resulting crystals from the ethylene glycol aqueous solution containing the soluble impurities.
U.S. Pat. No. 4,745,211 and Japanese Patent Laid-Open Publication No. 110650/1989 describe methods of purifying crude naphthalenedicarboxylic acid comprising the steps of causing impure 2,6-naphthalenedicarboxylic acid to react with ethylene glycol in an amount of at least 2 mol based on 1 mol of the 2,6-naphthalenedicarboxylic acid in the presence of catalytic amounts of a tertiary amine and an added titanium-containing compound as an esterification catalyst to prepare bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate; crystallizing the bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate; and recovering the purified bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate by crystallization. No impurities are removed by distillation.
Another method of purifying crude naphthalene dicarboxylic acid, optionally employing diglycol esters, is disclosed in U.S. Pat. No. 6,100,374 and comprises the steps of mixing crude naphthalene dicarboxylic acid and an alcohol aqueous solution, heating the resulting mixture to esterify a part of the naphthalene dicarboxylic acid and thereby give a naphthalene dicarboxylic acid ester, dissolving the naphthalene dicarboxylic acid ester in the alcohol aqueous solution; then contacting aldehydes, which are contained in the crude naphthalene dicarboxylic acid, with a sulfite to give aldehyde adducts and dissolving the aldehyde adducts in the alcohol aqueous solution; and subsequently crystallizing the naphthalene dicarboxylic acid and the naphthalene dicarboxylic acid ester and separating the resulting crystals from the alcohol aqueous solution.
U.S. Pat. No. 5,262,560 describes a process for purifying 2,6-naphthalenedicarboxylic acid which proceeds by preparing and purifying dimethylnaphthalene dicarboxylate. In particular, the process comprises the steps of: causing 2,6-naphthalenedicarboxylic acid to react with methanol in an appropriate reaction region to prepare a reaction mixture containing dissolved dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate; cooling the reaction mixture to a temperature of not higher than about 40xc2x0 C. to crystallize major parts of the dissolved dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate; fractionating the thus crystallized dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate from the reaction mixture solution; heating the thus fractionated dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate in a recrystallization solvent to a temperature high enough to dissolve at least a part of the dimethyl 2,6-naphthalenedicarboxylate and substantially all of the monomethyl 2,6-naphthalenedicarboxylate; recrystallizing the dimethyl 2,6-naphthalenedicarboxylate, which has been dissolved in the recrystallization solvent, at a temperature at which a major part of the monomethyl 2,6-naphthalenedicarboxylate is held in the recrystallization mother liquor; and fractionating the thus recrystallized dimethyl 2,6-naphthalenedicarboxylate from the recrystallization mother liquor.
Japanese Patent Laid-Open Publication No. 173100/1995 describes a process for preparing high-purity 2,6-naphthalenedicarboxylic acid comprising the steps of dissolving coarse crystals of impurity-containing 2,6-naphthalenedicarboxylic acid in water in a supercritical or subcritical state; cooling the resulting solution to a temperature of not higher than 300xc2x0 C. to precipitate the acid crystals; and separating the purified crystals from the mother liquor at a temperature of 100 to 300xc2x0 C.
In addition to the problems involved in purifying the crude terephthalic acid and naphthalene dicarboxylic acid, the oxidation process used to produce the crude acid has also been the subject of considerable research. Thus, existing oxidation processes for the production of terephthalic acid and 2,6-naphthalene dicarboxylic acid normally involve dissolving the para-xylene or 2,6-dimethylnaphthalene in an aliphatic carboxylic acid, such as acetic acid, and then treating the solution with molecular oxygen in the presence of a suitable catalyst. Typically, such catalysts include mixtures of cobalt and manganese promoted with bromine. However the presence of both bromine and acetic acid at the high reaction temperature involved makes the system highly corrosive, requiring the use of titanium and high nickel alloys throughout the plant and thereby increasing the equipment costs.
For example, U.S. Pat. No. 6,114,575 describes a process for preparing 2,6-naphthalenedicarboxylic acid by the liquid phase, exothermic oxidation of 2,6-dimethylnaphthalene comprising adding to a reaction zone oxidation reaction components comprising 2,6-dimethylnaphthalene, a source of molecular oxygen, a solvent comprising an aliphatic monocarboxylic acid, and a catalyst comprising cobalt, manganese and bromine components wherein the atom ratio of cobalt to manganese is at least about 1:1 and the total of cobalt and manganese, calculated as elemental cobalt and elemental manganese added to the reaction zone, is less than about 0.40 weight percent based on the weight of the solvent added to the reaction zone; maintaining the contents of the reaction zone at a temperature and pressure sufficient to cause the oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid and the vaporization of at least a portion of the reaction solvent while maintaining a liquid phase reaction mixture; condensing the vaporized solvent and returning an amount of the condensed solvent to the reaction zone to maintain the amount of water in the reaction zone at no more than about 15 weight percent based on the weight of solvent in the reaction zone; and withdrawing from the reaction zone a mixture comprising 2,6-naphthalenedicarboxylic acid.
However, during the liquid phase oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid using bromine-promoted catalysts, such as described in U.S. Pat. No. 6,114,575, various unwanted by-products are usually produced. For example, trimellitic acid (TMLA) is produced by the oxidation of one of the rings of the 2,6-dimethylnaphthalene molecule. 2-Formyl-6-naphthoic acid (FNA), a result of incomplete oxidation of one of the methyl groups of the 2,6-dimethylnaphthalene molecule, is also produced. In the presence of bromine, as an oxidation promoter, bromination of the naphthalene ring occurs during the oxidation reaction and results in the formation of bromonaphthalene dicarboxylic acid (BrNDA). Additionally, loss of one methyl (or carboxylic acid) substituent during the oxidation reaction results in the formation of 2-naphthoic acid (2-NA). These and other unidentified by-products are undesirable because they contaminate the 2,6-naphthalenedicarboxylic acid.
To obviate the problems associated with bromine-promoted oxidation catalysts, various proposals have been made for bromine-free oxidation processes. For example, U.S. Pat. No. 4,334,086 discloses the bromine-free oxidation of p-xylene to terephthalic acid in the presence of not more than about 10 weight % of water and a catalyst comprising a mixture of cobalt and manganese salts, wherein aldehyde/acid impurities comprising partially oxidized species are recycled as oxidation promoters.
Recycling of some naphthalate esters to the oxidation step is disclosed in U.S. Pat. No. 5,587,508 wherein high boiling residues from the distillation of dimethyl-2,6-naphthalenedicarboxylate are used as additives in the oxidation of 2,6-dimethylnaphthalene. The process continues to teach the use of a bromine as promoter in the oxidation and teaches that the only advantage to recycle of the crude methyl ester residues is to increase the particle size of the precipitating crude 2,6-naphthalenedicarboxylic acid thereby expediting filtration.
According to the present invention, it has been found that crude aryldicarboxylic acids can be purified by esterification followed by distillation without any intervening chemical treatment such as hydrogenation or treatment with sulfite. In particular, it has been found that the esterified partial oxidation products formed during the esterification process have significantly lower boiling points than the esters of the dicarboxylic acid and hence can readily be separated by distillation. The resultant purified dicarboxylic acid esters can then be subjected to direct polyesterification to produce the required polyester resin. In addition, it has been found that the esterified partial oxidation impurities distilled from the esterification effluent can be recycled to the oxidation reactor where they act as oxidation promoters thereby optionally allowing for a bromine free oxidation process for substituted aryl hydrocarbons.
The preparation of pure diglycol esters of 4,4-biphenyl dicarboxylic acid is addressed in U.S. Pat. Nos. 5,374,707 and 5,847,070. While these patents demonstrate the utility of pure glycol esters, with low levels of diethylene glycol, in mixed polyesterification reactions, the process starts with pure dicarboxylic acids and addresses only the reduction in the level of diethylene glycol produced in the esterification process. The patents do not teach the use of diglycol esters in the purification of aromatic dicarboxylic acids.
In accordance with one aspect of the invention, there is provided a process for purifying an aryldicarboxylic acid, comprising the steps of:
i) reacting the crude aryldicarboxylic acid with a glycol to esterify at least part of the aryldicarboxylic acid and produce an esterification effluent containing an aryldicarboxylic acid ester;
ii) removing volatile impurities from said esterification effluent by distillation; and
iii) after step (ii), separating the aryldicarboxylic acid ester from said esterification effluent.
Preferably, the crude aryldicarboxlic acid is first produced by the additional steps of (iv) oxidizing a disubstituted aryl hydrocarbon in the presence of a transition metal catalyst to prepare a mixture comprising said crude aryldicarboxylic acid; and
(v) separating the crude aryldicarboxylic acid from said mixture.
Preferably, at least part of the volatile impurities removed in step (ii) is recycled to step (iv) to act as an oxidation promoter.
In accordance with a further aspect of the invention, there is provided a process for purifying naphthalenedicarboxylic acid, comprising the steps of:
i) mixing crude naphthalenedicarboxylic acid with an aqueous solution of an alcohol;
ii) heating the mixture produced in step (i) to esterify a part of the naphthalenedicarboxylic acid and thereby give a naphthalenedicarboxylic acid ester, and
iii) dissolving the naphthalenedicarboxylic acid ester produced in step (ii) in the aqueous alcohol solution;
iv) then reducing the pressure of the aqueous alcohol solution to remove volatile species; and
v) subsequently crystallizing the naphthalenedicarboxylic acid ester from the aqueous alcohol solution and separating the resultant crystals from the aqueous alcohol solution.
In accordance with yet a further aspect of the invention, there is provided a process for preparing polyethylene naphthalate, comprising the steps of:
i) oxidizing 2,6-dimethylnaphthalene to produce an oxidation effluent comprising crude naphthalene dicarboxylic acid;
ii) separating the crude naphthalene dicarboxylic acid from said oxidation effluent;
iii) optionally, washing the crude naphthalene dicarboxylic acid with aqueous acetic acid;
iv) mixing the separated crude naphthalene dicarboxylic acid with an aqueous solution of ethylene glycol;
v) heating the resulting mixture to esterify at least part of the naphthalene dicarboxylic acid and thereby produce a naphthalene dicarboxylic acid ester;
vi) dissolving the naphthalene dicarboxylic acid ester in the aqueous glycol solution;
vii) then distilling the aqueous glycol solution produced in step (vi) to remove volatile impurities;
viii) subsequently separating the naphthalene dicarboxylic acid ester from the aqueous glycol solution remaining after step (vii), and
ix) subjecting the naphthalene dicarboxylic acid ester separated in step (viii) to a polycondensation reaction.